iMac Freezes at white screen during startup

You could try to use SafeBoot mode on startup, essentially you'd hold the Shift

key down on startup, perhaps you may need to login after waiting quite awhile,

then continue holding the shift key down until you see a traditional desktop.

And then use Disk Utility to verify HDD, & repair disk permissions.+ Restart.

Or if you have a system install-restore DVD, boot from that and have the

Installer's version of Disk Utility check verify repair the hard disk drive.

It can also repair disk permissions, too, from the booted version of D.U.

Then when it is done, choose the startup disk from installer menu options.

The DVD should eject, but if not, it should be ignored as a boot volume

after you've chosen the Mac HD as a boot system.

PS: If your computer is now running a system later than Snow Leopard,

where no DVD would be needed if it has a Recovery partition in the HDD

then you could see about booting into Recovery (command + R) to access

the later system's OS X Utilities, and choose Disk Utility to check/repair, etc.

These may help.

Good luck! 🙂

edited

same problem here, screen frozen after wake up with apple logo. Is first time the it happen in my 27" iMac.